1. Field of the Invention
The invention relates to a method and apparatus for controlling tilt in an optical disc device.
2. Background of the Related Art
Optical discs belonging to the CD and DVD families have been widely commercialized, and optical discs belonging to the BD (Blue-ray Disc) family are expected to be commercialized in the near future. The CD family includes optical discs, such as a CD-ROM (a read-only CD), a CD-R (a write-once CD) and a CD-RW (a rewritable CD), and the DVD family includes optical discs such as a DVD-ROM (a read-only DVD), a DVD-R (a write-once DVD) and a DVD-RW (a rewritable DVD).
Compared to CDs, DVDs have an insufficient optical margin since their track pitch is narrow and an objective lens provided in an optical pickup for DVDs has a high numerical aperture (NA). Thus, there is a need to perform a tilt control operation for controlling tilt of the objective lens by moving inner and outer sides of the objective lens vertically in opposite directions so that the recording surface of the optical disc is parallel to the objective lens.
If an optical disc is loaded in an optical disc device, such as a DVD recorder, the device performs focus and tracking servo operations, and also detects the optimal tilt offset value while varying the tilt of the objective lens. By controlling the tilt of the objective lens according to the detected tilt offset value, the optical disc device enables the recording surface of the optical disc to be parallel to the objective lens, so that data is correctly read from or written to the disc.
There are two tilt control modes, that is, a static tilt control mode and a dynamic tilt control mode. In the static tilt control mode, as shown in FIG. 1, an optimal tilt offset value (Tilt_Offset) is detected for an inner peripheral area (for example, a lead-in area) of an optical disc, and the detected tilt offset value is commonly used for all areas of the optical disc. Accordingly, the static tilt control mode is mainly used for read-only optical discs that have a relatively large optical margin.
In the dynamic tilt control mode, as shown in FIG. 2, optimal tilt offset values (Tilt_Offset #1, Tilt_Offset #2, and Tilt_Offset #3) are detected, respectively, for an inner peripheral area, a middle area, and an outer peripheral area of an optical disc, and the detected optical tilt offset values are selectively used for the inner, middle, and outer areas of the optical disc. Accordingly, the dynamic tilt control mode is mainly used for recordable optical discs that have a relatively small optical margin.
However, the following problem is caused, as shown in FIG. 3, if the dynamic tilt control mode is used while writing data on a rewritable optical disc, such as a DVD-RAM or a BD-RE (BD-rewritable), which is subject to defect management. To rewrite data, erroneously written in a defect area of the disc, in a spare area (SA) thereof, a tilt offset value (for example, Tilt_Offset #1 or #2) corresponding to the current position of an optical pickup is selectively used while performing random access to the spare area and the defect area repeatedly and alternately. An abrupt change in the tilt offset value in this procedure causes a fatal error in focus or tracking servo operations.